Question: Which of these numbers is prime? ${35,\ 53,\ 65,\ 78,\ 85}$
A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 53 are 1 and 53. The factors of 65 are 1, 5, 13, and 65. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 85 are 1, 5, 17, and 85. Thus, 53 is a prime number.